Family across Space

A preindustrial group of humans are forced to flee their planet of birth to escape a natural disaster. They use forbidden technology from the past.
The new wilderness planet has its own challenges. The technicians claim it's uninhabited. Some of the people from settlement disappear. Hunters find ruins and then traces of other humans.
What they don't know is that there is an interstellar civilization observing them, and interfering at times. Rubin looks for years after his wife's mother disappears. Then Rubin and Zarola disappear. Can they assimilate into the interstellar federation?
Rubin's daughter explores the wilderness across the river for years looking for traces of her parents until she is taken.
The skills of not so civilized humans are useful to a very civilized, highly technical, planet spanning federation. Especially their ability to fight, spy, and wage war.